Conventionally, an electronic still camera which captures an object image by a solid-state image sensing element such as a CCD and records it on a storage medium as a still image signal is known as an image sensing apparatus having a function of controlling the white balance of a color image signal. An electronic still camera of this type will be described below as an example.
In an electronic still camera of this type, to control the white balance of a sensed color image signal, three methods are available: a method of framing a sheet of white paper or the like within the entire field of the finder under the light source in a photographing place, photographing the sheet by manual operation in this state, detecting color temperature information using the sensed color image signal, and performing white balance control on the basis of the obtained color temperature information; a method of extracting achromatic image data from a photographed color image signal and determining the color temperature information of the light source from the information of the extracted image data; and a method of detecting color temperature information by a calorimetric sensor which detects the color temperature of ambient light.
If color temperature information is detected using a sheet of white paper or the like, the obtained color temperature information is almost accurate. However, the color temperature cannot predict in principle by a method of detecting color temperature information using a photographed color image signal, if achromatic data is not present in the image.
Also, if very little achromatic data is obtained from a color image signal, the detected color temperature information has a low reliability, i.e., accurate color temperature information cannot be obtained.